Indirect x-ray photodesorption of N215 and CO13 from mixed and layered ices

Abstract

X-ray photodesorption yields of 15N2 and 13CO are derived as a function of the incident photon energy near the N (~ 400 eV) and O K-edge (~ 500 eV) for pure 15N2 ice and mixed 13CO:15N2 ices. The photodesorption spectra from the mixed ices reveal an indirect desorption mechanism for which the desorption of 15N2 and 13CO is triggered by the photo-absorption of respectively 13CO and 15N2. This mechanism is confirmed by the X-ray photodesorption of 13CO from a layered 13CO/15N2 ice irradiated at 401 eV, on the N 1s -> π* transition of 15N2. This latter experiment enables to quantify the relevant depth involved in the indirect desorption process, which is found to be 30 - 40 ML in that case. This value is further related to the energy transport of Auger electrons emitted from the photo-absorbing 15N2molecules that scatter towards the ice surface, inducing the desorption of 13CO. The photodesorption yields corrected from the energy that can participate to the desorption process (expressed in molecules desorbed by eV deposited) do not depend on the photon energy hence neither on the photo-absorbing molecule nor on its state after Auger decay. This demonstrates that X-ray induced electron stimulated desorption (XESD), mediated by Auger scattering, is the dominant process explaining the desorption of 15N2 and 13CO from the ices studied in this work.

X-ray photodesorption of complex organic molecules in protoplanetary disks

“…The studies are conducted in the soft X-ray range, on the SEXTANTS beam line of the SOLEIL synchrotron facility. Two energy ranges were selected: (1) the 395-420 eV range, referred to as the N K edge region, where the photoabsorption is dominated by N-bearing species, that is, CH 3 CN,and (2) the 525-560 eV range, referred to as the O K edge region, where the photoabsorption is dominated by O-bearing species, that is, H 2 O or CO. Consequently, selective photoexcitation of CH 3 CN, H 2 O, or CO enables us to study possible indirect desorption mechanisms that have been highlighted in previous studies (Basalgète et al 2022). X-ray photodesorption yields extrapolated to the 0.4-10 keV range and averaged over different attenuated X-ray emission spectra of PMS stars, referred to as astrophysical yields, were derived in order to facilitate the implementation of X-ray photodesorption in astrochemical models.…”

“…The TEY is sensitive to the changes in the molecular composition near the ice surface with the ongoing irradiation, that is, with the photon fluence (expressed in photons cm −2 ), and it can be assimilated to the X-ray absorption spectrum of the studied ices. The ice depth probed by the TEY measurements is estimated to be a few tens of ML based on studies of water ice (Tîmneanu et al 2004) and of CO/N 2 ices (Basalgète et al 2022).…”

“…The beam was sent at a 47 • incidence relative to the normal of the substrate surface, and the spot area at the surface was ∼0.1 cm 2 . The calibration of the energy scale was performed similarly to what is described in Basalgète et al (2022). Namely, in the N K edge region, a TEY was measured on a pure N 2 ice at 15 K, and the TEY feature corresponding to the N 1s → π * (v ′ = 0) transition of N 2 was set to 400.868 eV according to Chen et al (1989).…”

“…The coefficient k X was calibrated on N 2 . k N 2 relates the QMS current to a calibrated number of N 2 molecules desorbed during TPD experiments (see Basalgète et al 2022 for more details of the calibration procedure). The factor k X associated with other neutral species was derived from k N 2 by taking into account (i) the relative differences in the electron-impact ionization cross sections between N 2 and the species X and (ii) the differences in the QMS apparatus function between m/z(N 2 ) and m/z(X).…”

“…The values of the yields in units of absorbed photons are also provided in Sect. 4 based on a similar method as in Basalgète et al (2022), assuming that up to 30 ML of the ice contribute to the desorption, and without taking the dilution of CH 3 CN into account. These yields can easily be extrapolated to environments other than protoplanetary disks.…”